The general principles of ICP-MS are well known. ICP-MS instruments provide robust and highly sensitive elemental analysis of samples, down to the part per trillion (PPT) range and beyond. Typically, the sample is a liquid solution or suspension and is supplied by a nebuliser in the form of an aerosol in a carrier gas; generally argon or sometimes helium. The nebulised sample passes into a plasma torch, which typically comprises a number of concentric tubes forming respective channels and is surrounded towards the downstream end by an helical induction coil. A plasma gas, typically argon, flows in the outer channel and an electric discharge is applied to it, to ionise some of the plasma gas. A radiofrequency electric current is supplied to the torch coil and the resulting alternating magnetic field causes the free electrons to be accelerated to bring about further ionisation of the plasma gas. This process continues until a steady plasma state is achieved, at temperatures typically between 5,000 K and 10,000 K. The carrier gas and nebulised sample flow through the central torch channel and pass into the central region of the plasma, where the temperature is high enough to cause atomisation and then ionisation of the sample.
The sample ions in the plasma next need to be formed into an ion beam, for ion separation and detection by the mass spectrometer, which may be provided by a quadrupole mass analyser, a magnetic and/or electric sector analyser, a time-of-flight analyser, or an ion trap analyser, among others. This typically involves a number of stages of pressure reduction, extraction of the ions from the plasma and ion beam formation, and may include a collision/reaction cell stage for removing potentially interfering ions.
A problem encountered with the above analysers, especially relatively low mass resolution devices such as quadrupoles, is the presence in the mass spectrum of unwanted artefact ions which interfere with the detection of some analyte ions. The identity and proportion of artefact ions depends upon the chemical composition of both the plasma support gas and the original sample. The interfering ions are typically argon-based ions (such as Ar+, Ar2+, ArO+), but may include others, such as ionised metal oxides, metal hydroxides or, depending on the matrix of the solution, molecules including matrix ions, e.g. ArCl+ or ClO+ in an HCl (hydrochloric acid) solution. The collision/reaction cell is used to promote ion collisions/reactions with a gas which is introduced into the cell, whereby the unwanted molecular ions (and Ar+) are preferentially neutralised and pumped away along with other neutral gas components, or dissociated into ions of lower mass-to-charge ratios (m/z) and rejected in a downstream m/z discriminating stage.
A collision cell is a substantially gas-tight enclosure through which ions are transmitted and it is positioned between the ion source and the main mass analyser. A collision/reaction target gas, such as hydrogen or helium, among others, is supplied into the cell. The cell typically comprises a multipole (a quadrupole, hexapole, or octopole, for example), which is usually operated in the radio frequency (RF)-only mode. Generally speaking, the RF-only field does not separate masses like an analysing quadrupole, but has the effect of focusing and guiding the ions along the multipole axis. The ions collide and react with molecules of the collision/reaction gas and, by various ion-molecule collision and reaction mechanisms, interfering ions are preferentially converted to non-interfering neutral species, or to other ionic species which do not interfere with the analyte ions.
An additional technique for discriminating against artefact or reaction product ions which pass out of the collision cell is by kinetic energy discrimination. The principle of this technique is that larger, polyatomic interfering ions will have a larger cross section for collisions in the collision cell, so generally lose more kinetic energy than analyte ions. By running a downstream device, such as the analysing quadrupole, or merely an electrically biased aperture, at a more positive potential than that of the collision cell, a kinetic energy barrier is provided. The more energetic analyte ions can overcome this barrier, while the collision cell product ions are impeded.
Some examples of collision cells using multipole rods are as follows. U.S. Pat. No. 5,767,512 relates to the selective neutralisation of carrier gas ions with a charge transfer gas. WO-A1-00/16375 relates to the use of a collision cell to selectively remove unwanted artefact ions by causing them to interact with a reagent gas. U.S. Pat. No. 6,140,638 relates to the operation of the collision cell with a pass band. U.S. Pat. No. 5,847,386, U.S. Pat. No. 6,111,250, and US-A1-2010/0301210 relate to the use of a DC axial field gradient on the rods in the collision cell. U.S. Pat. No. 5,939,718 and U.S. Pat. No. 6,417,511 relate to various assemblies of more than one multipole or a multipole and a ring stack. U.S. Pat. No. 5,514,868 and U.S. Pat. No. 6,627,912 relate to kinetic energy filtering methods.
In view of the above, it would be desirable to provide an alternative and/or improved collision cell multipole which can efficiently transmit analyte ions while reducing or preventing the passage of interfering species towards a downstream mass analyser. The invention aims to address the above and other objectives by providing an improved or alternative multipole and associated method.